Electrical currents (bearing currents) can occur in bearings in machines which significantly reduce the lifetime of said bearings. Bearing currents are electrical currents which occur in roller bearings or plain bearings of electrical machines.
They are caused by electrical voltages (bearing voltages) which arise as a result of electrical or magnetic stray fields within the machine or foreign voltages which come from outside and flow across the machine. The current flow occurs as soon as the bearing voltage lies above the breakdown voltage of the lubricant.
Examples of the negative affects of bearing currents are as follows                Grease burning (reduction of the remaining lubrication capability)        Pit formation in the raceways and in the roller elements        And in extreme cases, as consequential damage resulting from this: Formation of rifling in the raceways.        
These bearing currents have been a known phenomenon for decades. They are not able to be detected directly in the field and lead to significant expense for users or to high warranty costs for manufacturers. There is thus great interest in a measurement method or sensors which measure bearing currents and are able to evaluate them in an informative manner.
Bearing currents on electrical machines, especially during operation with power electronics, can reduce the lifetime of the motor bearing to a fraction. According to the existing prior art bearings damaged by electrical bearing currents are only noticed and replaced when the damage becomes apparent e.g. when noise develops or bearing grease is burned. This often leads to system shutdowns, which are enormously costly.
A major problem in operating the bearing is thus that of recognizing the predicted time of the failure and thereby the optimum time for replacing the bearing. Reacting too early means unnecessarily high maintenance costs; reacting too late means that the user incurs system shutdown costs.
Bearing currents are currently detected by the complicated method of classical bearing current measurement, which makes it necessary to reconstruct the motor several times (dismantle and rebuild it), or detects them quantitatively by means of direct voltage measurement by contact brush. The frequency range covered in this case typically lies in the single or two-digit MHz range. A higher frequency range is not possible because of the boundary conditions of the installation. Furthermore permanent monitoring is only possible to a restricted extent.
Measurement via the emitted electromagnetic field is very prone to misinterpretation through coupled-in interference and barely allows quantification of the bearing currents. Despite the great efforts made, no suitable solution has been found over the years. According to the prior art, bearing currents are thus only to be quantified by experts. In such cases however the information provided is only meaningful to a restricted extent, because of the low frequency range, see for example a patent application submitted by the applicant under the file reference PCT/EP2010/001150 entitled “Verfahren zur Detektion von Plasma-Lagerströmen (Method for detection of plasma bearing currents)”, or the sensitivity of the measurement of the emitted electromagnetic field to external interferers.
Sensor systems already exist for measuring electrical bearing currents in electrical machines (motors and generators).
In the classic measuring technique the bearing is isolated in relation to the motor housing. This isolation is bridged with a line. The bearing current can be measured for example with a current probe via this line.
A further option is described in publication DE 10 2005 027 670, indirect measurement via the bearing voltage and the calculation of the bearing current from the rapid voltage collapses during spark discharges. Measurement of the electromagnetic fields in the vicinity of the motor is also known for example from publication WO 2007106015.
A remedy for bearing currents and the bearing damage caused thereby is often associated with significant costs and can also only be assessed with difficulty in respect of whether it will be adequate. Cost-intensive measures have in the past in some cases still not led to the desired objective.
Classical bearing current measurement technique is expensive in terms of money and time. The motors must be dismantled and rebuilt in order to make the measurement possible.
Measurement via the bearing voltage generally requires coupling via contact brushes. These contact brushes are not maintenance-free and/or are expensive and thus not suitable for long-term monitoring. The installation of a contact brush is also in practice often only accepted for a short time by customers.
Measurements in the GHz range are difficult to implement since line lengths of individual wires must be very short.
The measurement of electromagnetic fields is generally sensitive to interference pulses from the environment, e.g. from switching of converters. As a result measurements are not recognized by engineers. Furthermore amplitude information is hardly possible since the attenuation is not defined by the propagation.
The object of the invention is to specify a solution for the problems cited above. A measurement method and a device are to be specified which allow a better assessment of the bearing currents. Furthermore a method and a device are to be specified which realize a non-contact measurement of bearing currents. The object of the invention is also to specify a measurement sensor and a method which is suitable for long-term measurement of bearing currents.